1. Field of the Invention
The present invention relates to a method for computing an energy loss generated in a viscoelastic material and a method of evaluating an energy loss of a golf ball by using the method of computing the energy loss generated in the viscoelastic material. More particularly, the present invention is intended to compute the energy loss generated in the viscoelastic material during its dynamic deformation behavior at regular intervals to improve analysis accuracy of the dynamic deformation of the viscoelastic material. In particular, the present invention is intended to evaluate the energy loss generated inside the golf ball when it is hit.
2. Description of the Related Art
A viscoelastic material represented by a macromolecular material such as rubber or elastomer is widely applied to various products such as tires, balls used in sport, rolls for printing machines.
To save cost and time required to make a trial manufacture, simulation is made in various industrial fields to develop various products composed of the viscoelastic material. For example, to estimate the restitution performance of a golf ball, the present applicant proposed a method of simulating actual hitting tests by an analysis using a finite element method, as disclosed in Japanese Patent Application Laid-Open No. 2002-55034.
To improve the restitution performance of the golf ball, in one proposal, attention is given to an energy loss generated inside the golf ball when it is hit. More specifically, when a hitting object impacts (collides with) the golf ball, the golf ball deforms and separates from the hitting object, thus generating a repulsion. It is known that the restitution coefficient of the golf ball at this time is greatly affected by the energy loss generated inside the golf ball in the collision between the hitting object and the golf ball. Investigations are made to compute the energy loss generated in an to-be-analyzed object (hereinafter referred to as to-be-analyzed object) such as the golf ball composed of the viscoelastic material by simulations.
More specifically, in the process where the viscoelastic material is subjected to a stress, generates a strain, and returns to its original state, there is a relationship (stress-strain curve) between the stress and the strain, as shown in FIG. 14. The curve of FIG. 14 indicating the relationship between the stress and the strain goes momently from a point O to a point A where the absolute value of the strain is maximum and returns to the point O. At that time, a difference (a portion shown with vertical lines) is generated between a work amount done while the curve goes from the point O to the point A and a work amount done while the curve returns from the point O to the point A. By using the value of the difference (area shown with vertical lines) between the above-described two work amounts, it is possible to find the energy loss generated in the viscoelastic material.
To find a difference L between the two work amounts, computations are performed to determine a sum total L1 (work amount indicated with vertical lines of FIG. 15A) of the work amount done during the movement of the stress-strain curve from the point O to the point A and a sum total L2 (work amount indicated with vertical lines of FIG. 15B) of the work amount done during the movement of the stress-strain curve from the point A to the point O.
The difference L (area shown with vertical lines in FIG. 14) between the two work amounts is computed by using the sum total L1 of the work amount done during the movement of the stress-strain curve from the point O to the point A and the sum total L2 of the work amount done during the movement of the stress-strain curve from the point A to the point O. The energy loss can be computed by the product of the value of the work amount difference L and the volume of each element in its initial configuration. As described above, in the conventional art, the energy loss is computed by using the area (loop area) in the stress-strain curve, called hysteresis loss, showing the relationship between the stress and the strain.
However, the above-described method computes the energy loss of a to-be-analyzed object at the time of termination of the dynamic deformation behavior thereof when the to-be-analyzed object almost converges its deformed state converges and returns to its original state. The energy loss of the to-be-analyzed object in the entire stages of its deformation behavior is sufficient for estimating the restitution performance of the golf ball. However, the energy loss computed momently during its deformation behavior can be utilized to evaluate a player's feeling when the player hits the golf ball in addition to the evaluation of the restitution performance.
In the case where the viscoelastic material does not return to its original state from its deformed state, the strain does not return to zero and the loop does not have a closed state in the above-described stress-strain curve. In this case, the above-described method of computing the energy loss by using the area of the loop is incapable of computing the energy loss. The above-described method is incapable of computing the energy loss unless an analysis finishes, thus being incapable of evaluating the energy loss in the course of the deformation.